Spurred on by the evolution of drug-resistant microbes and the need for simple technologies to produce clean drinking water in the developing world, many researchers have developed new microbial disinfection techniques. In ES&T’s best technology paper from 2011, Jae-Hong Kim and his colleagues at the Georgia Institute of Technology demonstrate a new chemical surface that kills microbes by harvesting visible light and converting it to higher-energy ultraviolet wavelengths (Environ. Sci. Technol., DOI: 10.1021/es200196c).
Researchers have long relied on ultraviolet light to kill microorganisms. Solar water disinfection (SODIS) is an established technique. In SODIS, placing water in plastic bottles in direct sunlight for several hours takes advantage of UVA radiation’s ability to kill pathogenic bacteria. UVC light, between 220 and 290 nm, can disinfect even faster. Germicidal lamps use UVC to kill microorganisms in water and in wastewater treatment plants.
Kim (pictured with graduate student Ezra Cates) and his team wondered if there might be a simple way to convert lower-energy visible light into this higher-energy UVC radiation. They were inspired by upconversion, a physical process in which an atom absorbs more than one low-energy photon and then emits one higher-energy photon. Most other studies of upconversion have focused on transforming infrared radiation into visible light for applications such as bioimaging and solar cells.
Upconversion materials often contain lanthanide ions. Previous studies by other researchers had shown that yttrium laser crystals doped with praseodymium (Pr3+) ions could emit photons in the UVC range. So Kim and his colleagues used that material as a starting point for new ones. The researchers developed yttrium orthosilicate crystals doped with Pr3+ and used emission spectroscopy to characterize their photochemical properties. They found that to boost the material’s UVC emission efficiency, they needed to add gadolinium (Gd3+) and lithium (Li+) ions.
The researchers then tested the materials’ disinfection abilities. Because visible-to-UV upconversion processes are not very efficient, Kim decided that attempting to disinfect drinking water was not the best system for a first test. Instead, he and his colleagues coated their materials onto glass slides to construct antimicrobial surfaces. This two-dimensional surface would more fully expose the organisms to the emitted UV light.
In one set of experiments, the researchers dipped coated slides for a day into bioreactors containing Pseudomonas aeruginosa bacteria. No bacteria grew on the surfaces of slides that were in bioreactors illuminated by fluorescent lamps, but the microbes did grow on nonilluminated slides. The researchers also tested their materials on spores of Bacillus subtilis. They coated the slides with the spores and then illuminated them for 10 days. The surface inactivated 90% of the bacterial spores, Kim says.
The research shows “sophisticated, out-of-the-box thinking,” says Richard Luthy of Stanford University, who was not involved in the study. “This material could provide really interesting applications from bathroom tiles to solar disinfection.” He notes that, as with any new technology, questions remain about optimizing cost, efficiency, and reliability.
The crystalline materials are already inexpensive, costing less than $3 per m2, Kim says. He and his colleagues have already made bathroom tiles with the materials. Kim’s next step is to improve the efficiency of the upconversion process. He and his colleagues are tweaking the material compositions and doing crystallographic studies to enhance the efficiency of upconversion.
“My whole motivation for this research is to enhance water treatment in the developing world,” he says. According to the United Nations, nearly 900 million people lack access to sufficient safe drinking water. Kim hopes that a version of his material might speed solar disinfection of drinking water.
“I think it’s an excellent study,” says Pedro Alvarez of Rice University, who has collaborated with Kim on another project. Particularly in the developing world, “we need to move toward water treatment technologies that use less energy, require less infrastructure, and generate less residuals,” he adds. “I think this is a technology that can move us in this direction.”
The authors declare no competing financial interest.